Abstract

The objective of this tutorial is to generate a symmetric tilt grain boundary in LAMMPS. This tutorial can serve as a precursor to more advanced techniques, whereby in-plane translations and atom deletion criteria are used to sample a large number of potential structures to find the global minimum energy grain boundary structure[1][2].

Methodology

The following input script shows how multiple translations and an atom deletion criteria are used to calculate the minimum energy structure. This input script for LAMMPS[3] can be called with a command of the form, "lmp_exe < input.script." This script contains loops over x-translations, z-translations, and atom overlap distances (an atom is deleted when an atom pair with a nearest neighbor distance is less than this distance). The unique minimum energy structures are saved as a dump file with the energy appended to the filename in a new folder specified by the 'gbname' variable. The dump files can then be easily scanned through for the global minimum energy structure. Simulations performed on Aug 2014 LAMMPS version.

The grain boundary energy is calculated as 567.57 mJ/m^2. The final configuration is stored in a restart file for future use in predicting properties of grain boundaries. Additionally, files with atomic coordinates, energies, and centrosymmetry values are dumped for post-processing.

Images and movies

Figure 1 shows a movie of minimization of the grain boundary structure for the input script show above. Figure 2 shows a movie of minimization of the grain boundary structure for a modification of the input script above. Here, the "delete_atoms overlap 0.35 lower upper" command has been modified to "delete_atoms overlap 1.5 lower upper". This eliminates two atoms that are too close to each other near the grain boundary interface.

Figure 3 shows the image corresponding to the input script shown above. Notice how there are two atoms that are very close to each other at the boundary in Figure 3 (shown in red, colored by potential energy, i.e., high energy atoms). The final structures are shown in Figures 4 and 5 for the input script above and the modified input script, respectively. Interestingly, with this potential, the structure in Figure 4 is actually the lower energy grain boundary structure. The different structures for this one simple grain boundary may impart slightly different properties as well. Often, the convention is to use the minimum energy grain boundary structure for properties. However, some recent research has also explored metastable higher energy grain boundary structures and their impact on properties as well.

Miscellaneous

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Acknowledgments

M.A. Tschopp would like to acknowledge funding provided under an NSF graduate fellowship for the initial work. Continued funding for investigating structure-property relationships in grain boundaries under the NEAMS (Nuclear Energy Advanced Modeling and Simulation) program is also acknowledged.